Method of inspecting a cathode-ray tube



SUKUH MJU 'l T. S. NOSKOWCZ METHOD OF INSPECTING A CATHGDE-RAY TUBE June 11, 1957 Filed Aug. l5, 1955 Nm Sm E Dn Y B w H IS ATTORNEY.`

METHOD F INSPECTIDIIEG A CATHODE-RAY TUB Theodore S. Noskowicz, Wooddale, lll., assignor to The Rauland Corporation, a corporation of Illinois Application August 15, 1955, Serial No. 528,380

3 Claims. (Cl. 88-14) This invention relates to image-reproducing devices such as cathode-ray tubes, and more particularly to a method for inspecting such devices during the manufacturing process.

The use of aluminized-screen cathode-ray tubes in commercial television receivers is well known. Among the important advantages secured by aluminizing the rear surface of the luminescent screen are the presentation of a brighter picture by prevention of light loss from reflection back into the tube, reduction of damage to the luminescent screen resulting from ion burns, and prevention of the accumulation of charges in the vicinity of the screen by connecting the aluminum film to the conductive coating in the flared portion of the tube. In the manufacturing process, the envelope is normally inverted and a fluorescent screen comprising minute phosphor particles is deposited on the interior surface of the face plate; this phosphor layer has a thicknes of the order of l0 to 20 microns. Next a thin nitrocellulose film is deposited on the phosphor layer. Over the nitrocellulose lm an aluminum layer of the order of half a micron in thickness is deposited by evaporation, and the tube is then passed through a heat-applying device to bake out the nitrocellulose film. These steps in the manufacturing process are well-known and widely used; they constitute no part of the invention.

Where even minute openings or discontinuities are present in the nitrocellulose film, the vaporized aluminum may have direct access to the phosphor layer; the resultant imperfections appear as dark spots when the fluorescent screen is excited by a cathode-ray beam. Correction of the imperfections necessitates removal of the phosphor layer, the nitrocellulose film, and the aluminum layer; the manufacturing process must be commenced again at the point of depositing the phosphor layer. A substantial loss is thus occasioned, both labor and material; in an attempt to obviate this loss an inspection of the nitrocellulose lm is made before the aluminum layer is deposited. If discontinuities or other imperfections in the film are noticed during this inspection, the imperfections are corrected before the aluminum layer is added, and the manufacturing process continues in its normal sequence. Until now the common method of inspecting the nitrocellulose film has been to direct a bright light through the walls of the cathode-ray tube, upwardly through the faceplate, while the film is inspected from a point above the faceplate; in this manner some of the flaws can be detected. However, this method is far from satisfactory, and a large number of picture tubes have substantial defects in the nitrocellulose film which, in such an inspection, either are not visible, or their extent is not apparent. These tubes are passed on to the aluminizing stage, and the imperfections only become apparent after the aluminum has entered the fluorescent screen, necessitating removal of the materials from the faceplate of the cathode-ray tube.

It is an object of the invention to provide an improved method for inspecting a thin nitrocellulose lm such as that applied to the fluorescent screen during the manufacture of an aluminized-screen cathode-ray tube.

nited States Patent O ICC The inventive method comprises illuminating one surface of the film with light polarized in a plane substantially parallel to that of the film, at an incidence angle of greater than 45, while the opposite surface of the film is observed from any convenient vantage point. Incidence angle, as used herein, is the acute angle between the impinging light rays and a line from the point of incidence, perpendicular to the plane of the nitrocellulose film.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the 4following description taken in connection with the accompanying drawings, in the several iigures of which like reference numerals identify like elements and in which:

Figure l is a representation, partly schematic and partly in section, useful in understanding the inventive method of inspecting nitrocellulose films on fluorescent screens;

Figure 2 is a representation, partly schematic and partly in section, useful in understanding the advantages of the invention;

Figure 3 is a photograph of a nitrocellulose film as illuminated by conventional inspection means; and

Figure 4 is a photograph of the same film as that shown in Figure 1 but illuminated in accordance with the invention.

Figure l shows the flared end portion of a cathode-ray tube having a side wall 11 and a faceplate 12. A fluorescent screen 20 and a nitrocellulose film 19 are affixed to the inner surface of faceplate 12; the screen and the film are so thin with respect to the thickness of faceplate 12 that their thicknesses are greatly exaggerated for purposes of illustration. A light source 13 is positioned below the level of faceplate 12, and, in accordance with the invention, a polarizing filter 14 is interposed between light source 13 and side wall 11 of the tube. Filter 14 is constructed so that light rays 15 passing through filter 14 are polarized in a plane substantially parallel to the plane of faceplate 12. Incident light ray 15 is divided upon striking film 19 at point 16; reflected ray 17 is directed downwardly, and ray 18 is refracted in film 19, fluorescent screen 20, and faceplate 12 and directed toward point X, which may be any convenient observation point above faceplate l2. Incident light ray 15 and a line 24, constructed perpendicular to the plane of film 19 at point 16, define an acute angle the angle of incidence of light ray 15.

Figure 2 shows, in exaggerated thicknesses, nitrocellulose film 19 and phosphor layer 20. Light ray 15 impinges on nitrocellulose film 19 at point 16, and most of the energy in light ray 15 is directed as shown in reflected ray 17. The remainder of ray 1S traverses film 19, being refracted therein, and impinges on phosphor layer 20; were the energy of refracted ray 18 not scattered in phosphor layer 20, as it is, it would continue toward the observation point, as shown in the drawing. In the event that film 19 is discontinuous, as for example due to a pinhole or other imperfection 22, light ray 21 passes through aperture 22 and impinges directly on phosphor layer 20; the projection of light ray 21 is shown along path 23.

The advantages :of the invention and the manner in which they are 'achieved are best described in connection with Figure 2. Although the reasons underlying the improvement realized by employing the invention are not fully understood, the use -of polarized light in accordance with the invention insures =a marked increase in the contrast observed between continuous and discontinuous areas 'of nitrocellulose film aflixed to the fluorescent screen of a cathode-ray tube faceplate. Light rays emanating from source 13 and passing through polarizing filter 14 are polarized in a plane substantially parallel to that of film 19, the horizontal plane in Figure 2. It is thought that when light ray 15 strikes film 19 at point 16, substantially `all of the energy represented by light polarized in the same plane as that of film 19 is reflected `along path 17, 'and only the small amount of light residually polarized in the vertical plane traverses film 19 and strikes phosphor layer 20. In the absence of the phosphor layer, the light traversing film 19 would be refracted land continue along path 18, but actually this energy is scattered as the light strikes phosphor layer 20; some little illumination of phosphor layer 20 may be observed from point X when light ray 15 strikes point 16. However, when light ray 21 passes through aperture Z2 of film 19, Iall of the energy in light ray 21 reaches phosphor layer 20; the polarized ray is scattered in all directions, giving the appearance of a bright spot when viewed from above. The invention, by enabling substantially all the light energy to pass through apertures in film 19 while effecting reflection of substantially all the energy from the continuous portions, provides a very high degree of Icontrast between the continuous and discontinuous sections of film 19. Best results have been obtained with the polarized light striking film 19 at very lhigh angles of incidence, i. e., of the Iorder of 60 or more. The improvement in contrast afforded by the invention disappears entirely when the plane of polarization effected 'by filter 14 is rotated through 90, lor when the angle of incidence is decreased to zero.

The improvement attributable to use of the polarized inspection light of the invention appears related to the behavior of natural light when directed upon a reflecting surface at the polarizing angle, at which the reflected and refracted rays form a 90 angle. At the polarizing angle all of the energy in the incident wave vibrating in a plane parallel to the plane of incidence is refracted, and only some of the energy contained in light vibrating in a plane perpendicular to the plane of incidence is reflected. It -appears thfat when ordinary unpolarized light, such as that used in accordance with the prior art, is employed for film inspection, "a much greater percentage of the incident light traverses the continuous areas lof the nitrocellulose film than does the polarized light employed in accordance with the invention. This accounts for the much greater contrast observed when inspecting nitrocellulose films with polarized light than with the conventional method of scrutinizing the film when illuminated by unpolarized light. Obviously, this contrast can be increased as filter 14 is made more efiicient. Theoretically, it is possible to polarize all the light energy in one plane, and thereby to refiect all the light energy from the continuous portions of the film. In practice the filter is necessarily less than perfect, and some depolarization occurs as incident ray 15 strikes side wall 11; however the amount of energy represented by light polarized in a plane perpendicular to that of the faceplate is insignificant as cornpared with the improvement realized.

Figure 3 shows a faceplate having a nitrocellulose film and phosphor layer deposited on its inner surface and illuminated by ordinary unpolarized light emanating from a conventional source and passing through the walls of the cathode-ray tube; some discontinuities can be observed in the photograph of Figure 3. When, in accordance with the invention, polarizing filter 14 is inserted between the light source and the cathode-ray tube, the discontinuities and other imperfections are greatly emphasized, as clearly shown in the photograph of Figure 4. IFigures 3 and 4 were taken under identical conditions, with the single exception that the inspection Ilight employed in Figure 4 was polarized in a plane substantially parallel to that of the faceplate of the picture tube. A substantial difference in contrast between the continuous and discontinuous areas of the nitrocellulose film is evident upon the most casual observation of Figures 3 and 4. A hole or tear 25 which is visible in Figure 3 is much more clearly shown in Figure 4, where the polarized light emphasizes the defect. Hole 26 is barely visible in Figure 3, yet is clearly shown in Figure 4, wherein the inspecting means of the invention is utilized. Holes 27 and 28 are hardly discernible in Figure 3, yet in Figure 4 the same holes appear in sharp contrast to the adjacent continuous portions of the nitrocellulose film. The bright area 29 in the central portion of both Figures 3 and 4 represents a section where the nitrocellulose lm is thinner than in the adjacent areas. In Figure 3 the glare caused by thickness variation 29 masks many of the discontinuities, lsuch as holes 27 and 28. But when, in accordance with the invention, polarized light is used to illuminate the film as in Figure 4, not only are thickness variations more accurately portrayed but holes such as 27 and 28 are readily visible.

The invention provides an efficacious means for substantially increasing the contrast between the continuous and 4discontinuous areas of anitrocellulose lm applied to the fluorescent screen in the manufacture of aluminized-scre'en cathode-ray tubes. The invention enables fiaws to be detected more easily and their extent to be ascertained with greater certainty, and further enables minute flaws not visible under standard inspection techniques to be discovered. Detection of more iiaws permits their correction before the 4aluminum screen is applied over the nitrocellulose film, thus obviating the waste of labor and material attendant upon depositing an Ialuminum screen on an imperfect film.

While a particular embodiment of the invention has been shown and described, it is apparent that modifications and alterations may be made, and it is intended in the appended claims to cover all such modifications and alterations as may fall within the true spirit and scope of the invention.

I claim:

l. The method of inspecting a cathode-ray tube for discontinuities of a thin organic film applied to its fluorescent screen which comprises illuminating one surface of said film with light polarized in a plane substantially parallel to that of said film, and observing the opposite surface of said film from any convenient vantage point.

2, The method of inspecting a cathode-ray tube for discontinu-ities of a thin organic film applied to its fi-uorescent screen which comprises illuminating said film from one side thereof at an incidence angle of greater Ithan 60 degrees with light polarized in a plane substantially parallel to that of said film, and observingsaid film from any convenient vantage point on the opposite side of said film, whereby even minute defects in said film are visually emphasized and made readily apparent on the most casual observation.

3. The method of inspecting a cathode-ray tube for discontinuities of a nitrocellulose film deposited on -a fluorescent screen afiixed to the inner surface of a faceplate joined to the side walls of a cathode-ray tube envelope which comprises: directing light rays through said walls onto the inner surface of said film at substantially high angles of incidence; and polarizing said rays in a plane substantially parallel to said film, whereby fiaws and imperfections in said nitrocellulose film are visually emphasized and made readily `apparent upon the most casual observation from a vantage point on the side of said faceplate opposite said lm.

References Cited in the file of this patent UNITED STATES PATENTS 2,073,691 vGray Mar. 16, 1937 2,192,860 Bennett et al. Mar. 5, 1940 2,501,446 Justice Mar. 21, 1950 FOREIGN PATENTS 722,746 Germany July 20, 1942 

